Analog-to-digital converters (ADCs) allow for an analog input signal to be sampled into the digital domain. ADCs have found wide-spread use in communications, as it allows the digitized signals to be processed with powerful digital signal processing (DSP) techniques. As electronic ADCs have developed, uses in RF-wireless communications such as cellular telephony and software defined radio have been made possible. ADCs in RF-wireless applications typically have high resolutions because bandwidth restrictions require the use of dense signal constellations. Other common uses for ADCs include instrumentation, such as high-speed real-time oscilloscopes, medical imaging, and radar.
Electronic ADC's have made steady technological progress, but issues such as clock-jitter and internal parameter mismatches make it difficult for ADCs to maintain high resolution, typically measured in effective number of bits (ENOB), as the sampling rate increases. It is common for ENOB to fall 1 bit for every factor of 2 rate increase, for example, see U.S. Pat. No. 5,010,346 by Hamilton et al.
Photonic technology can be used to aid in creating faster ADCs. The performance improvement is due to various factors depending on the specific design, but may stem, for instance, from the ability to generate ultra-short pulses with ultra-low timing jitter in the optical domain. Although progress has been made in optical ADC technology, there are few experimentally verified cases where optical ADCs reach their expected potential. For instance, as it is shown in U.S. Pat. No. 5,010,346 by Hamilton et al., when multiplexing is used to reach high sample rates differences in the parameters of the optical-to-electrical (O/E) detectors and the subsequent electrical ADCs used to sub-sample the signal make the process of recombining the various ADC outputs into a single higher-rate output imperfect and can cause performance degradation. Also, there is a nonlinearity problem in many modulators used to transfer the RF signal into the optical domain. This nonlinearity problem can cause unwanted distortions in the signal. It can be managed by reducing the size of the modulation on the optical signal to much less than π radians, however this technique also reduces the inherent signal-to-noise ratio of the measured signal. Some of the aforementioned problems can be alleviated by using phase modulation, which is naturally linear, as opposed to amplitude modulation which is typically nonlinear, such as the sinusoidal response from a Mach-Zehnder modulator. Using phase modulation requires a phase-detector which can be implemented in several ways. For instance, Twichell et al. in U.S. Pat. No. 5,955,875 used a modulator with two optical outputs, digitized the two outputs, then applied an inverse transformation via a digital signal processor to re-construct the voltage applied to the modulator. The most common modulator to use is a dual-output Mach-Zehnder modulator, but that restricts the applied voltage to somewhat less than π radians (assuming NyQuist sampling). Such a dual-detection method is inherently less sensitive to optical power fluctuations than a single-detector design.
A full 2π modulation could be measured using coherent detection, but this tends to be more complex and expensive than other detection methods. A full modulation can also be measured using an optical hybrid that allows the measurement of both the in-phase and quadrature phase (I and Q) portions of the signal such as shown in U.S. Pat. No. 4,732,447 by Stephen Wright et al. Such a method was used for RF photonic links in “Coherent optical phase-modulation link,” IEEE Photonics Technology Letters, v. 19, no 16, pp 1206-1208, Aug. 15, 2007 by T. R. Clark and M. L. Dennis and for digitizing differential M-ary modulated optical signals in U.S. Provisional patent application Ser. No. 12/482,267 “System and method for data transmission over arbitrary medium using physical encryption,” filed Jun. 10, 2009.
Photonic ADCs often make use of low jitter optical and/or electrical signals, since low jitter results in higher ENOB performance especially when digitizing high frequency signals. One promising method of generating both low jitter optical and electrical signals is to use an opto-electronic oscillator (OEO) U.S. Pat. No. 6,567,436 by Yao et al. Other methods include mode-locked lasers.
Jitter in the sampling pulse train can be measured by integrating the phase-noise spectrum of the sampling pulse train around the repetition frequency, where typically the integration is performed over a spectrum of ½ the sample rate. In a NyQuist sampling system the sample rate is twice the highest frequency component of the signal to be digitized. Thus a 10 Giga-sample-per-second (10 Gsps) sampling train can digitize signals from 0-5 GHz and the relevant jitter bandwidth is integrated to 5 GHz.
In addition to the standard NyQuist sampling ADC there are times when under-sampling at lower frequencies can be useful. Under-sampling allows a high carrier frequency to be digitized with a sample rate much less than twice the carrier frequency, but the sample rate must still be at least twice the total bandwidth of the signal. The low jitter and small aperture time of mode-locked lasers can be helpful in these applications. If for instance a 5 GHz center frequency signal can be sampled with a 200 MHz sample rate, in which case the instantaneous measurement bandwidth is 100 MHz and the relevant jitter bandwidth should be integrated to 100 MHz.
What is needed is an optical ADC with high resolution. It should be insensitive to power fluctuations in the optical source, have a high linearity ideally even when being strongly modulated by more than π radians, and be capable of providing ENOBs even greater than the ENOB of the electrical ADCs employed in the system. Ideally the system should be compatible with the use of integrated optics. The system should have a self-calibration and self-monitoring function to optimize and monitor the system performance. The system should be scalable to high sample rates (10's of Gsps or more) and/or it should be able to operate in the under-sampling mode of operation where a high carrier frequency, for instance of several GHz to many 10's of GHz, can be digitized over a relatively small bandwidth using a low sample rate of at least twice the signal bandwidth. It is desirable if the same system can operate in both the NyQuist and under-sampling modes either simultaneously or by user selection, or in a mode where under-sampling is performed at two or more different sampling frequencies. This selection can be useful since a lower rate sampling mode may have higher resolution performance, but only for those class of signals for which it is capable of digitizing. The added functionality of switching between lower and higher rate sampling modes should come with relatively little added cost or complexity. It is also desired that several independent signals can be digitized using a single ADC system. These signals may require different sample rates and have different target ENOBs. It is desired that this added functionality come with only incremental increases in cost, size, and power consumption.